Metal cans which are a kind of food packaging containers have high mechanical strength and excellent heat resistance and air-tightness. With these advantageous characteristics, metal cans are able to store contents over a long period. In addition, they accept the contents at high temperatures and seal the contents in that high temperature state, and are able to store the contents in a heated state. Since they allow sterilization such as retorting easily, they acquire high reliability of safety and hygiene as the packaging containers. Furthermore, they have advantages of relatively easy for classifying and collecting the used cans. The term “retorting” referred herein signifies the heat treatment of cans filled with contents by high temperature steam or the like to sterilize the contents by heat.
Conventionally the metallic base materials for food cans adopt electrolytic chromium coated steel sheet (also referred to as “ECCS”), aluminum sheet, and the like. Since these metal sheets for containers are coated by solvent-type coating consisting mainly of a thermosetting resin not only to prevent corrosion but also to keep the flavor of contents (on the surface of the metal sheet facing the inner surface of the container) and to give decorative appearance and to protect printed face (on the surface of the metal sheet facing the outer surface of the container). The coating technique, however, has complex baking process and takes a long treatment time, and also raises a problem of discharging large amounts of solvent. To solve these problems, a current main stream of the technology applies lamination of thermoplastic resin film on the heated metal sheet, and many kinds of technologies have been provided as the lamination method. The term “lamination” referred to herein signifies that at least one side of the metal sheet is covered by a resin film.
For example, JP-A-5-156040 and JP-A-7-195617, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), describe a thermo-compression-bondable polyester film for lamination.
On the other hand, JP-A-3-212433 and JP-A-5-92535 describe a manufacturing method of metal sheet laminated with a thermo-compression-bondable polyester film, and further a manufacturing method of high-drawing ratio can from thus obtained laminated metal sheet.
With these conventional metal sheets coated with polyester resin, however, when the resin-coated surface faces outer side of a food can, the oligomer in the polyester resin often precipitates on the resin surface during the high temperature sterilization such as retorting. The precipitate gives appearance of white powder, which raises a problem of damaging the decorative appearance, (what is called the “white haze phenomenon”). Furthermore, during the retorting, there occurs a phenomenon of discoloration in clouding the resin layer itself, (what is called the “retort blushing phenomenon”). Therefore, the request for solving these decorative appearance problems on outer surface of the can has become strong.
The current main stream of preventive technology against white haze is the one to decrease the quantity of oligomer in the polyester resin. For example, JP-A-10-110046 discloses that the precipitation of oligomer is suppressed even in high temperature treatment such as retorting if only the oligomer quantity in the film is restricted to 1.3% by mass or less. The retorting applied to food cans is, however, generally a high temperature and longtime treatment at 120° C. or above for 90 minutes. As a result, at a level of 1.0% by mass or more of oligomer quantity given in Examples of the disclosure, the prevention of white haze is practically impossible. In JP-A-11-79189, there is disclosed a technology to suppress elution of ester oligomers in the polyester resin into water. Even with the technology, however, the maintaining of decorative appearance does not reach the satisfactory level.
Regarding the retort blushing phenomenon, which is another problem of decorative appearance, several improving technologies have been proposed. For example, JP-A-6-155660 proposes a technology to laminate a polyester resin on a metal sheet. Specifically, the patent publication discloses a technology to control the crystallinity in the resin layer contacting the metal sheet and the crystal orientation in the resin layer not-contacting the metal sheet. The technology was established by speculating the retort blushing phenomenon as follows. That is, since the rate of crystallization under the retorting of amorphous resin layer contacting the metal sheet differs between the dew-forming part and the not dew-forming part on the surface of the metal sheet, these parts become resin layers having different light refractive index and volume from each other, which induces scattering of light to give whitened surface appearance.
In JP-A-5-331302, it is speculated that the rate of crystallization of polymer is low during retorting, thus the crystals slowly grow to coarse grains, thereby causing the retort blushing phenomenon. Based on the speculation, the patent publication proposes the enhancement of the rate of crystallization of polymer during retorting to generate large numbers of fine crystals. Since, however, both technologies do not accurately grasp the mechanism of retort blushing phenomenon, maintaining decorative appearance is not sufficient, and they are not a kind of adequate improvement technologies.
The present invention has been perfected to solve the above problems relating to the outer surface of food cans, and an object of the present invention is to provide a resin-coated metal sheet which does not induce white haze phenomenon and retort blushing phenomenon even after retorting.